1. Field of the Invention
The present invention relates to a coolant composition capable of improving the fuel efficiency of an internal combustion engine, and a method of operating an internal combustion engine using the coolant composition.
2. Description of Related Art
As a coolant for cooling an automotive engine or the like, various products are known. Among these, water is preferable because it has the highest cooling performance as the coolant for an engine. However, pure water is frozen at 0° C. or lower. Because of the above-described reason, a coolant composition is used, in which a glycol such as ethylene glycol as a base for obtaining anti-freezing properties is diluted with water to obtain a required freezing point, and various additives for preventing deterioration of metal, rubber, resins, and the like, which are used for, for example, an engine and a radiator, are optionally added thereto.
However, when a glycol such as ethylene glycol is used, there is a problem in that the viscosity of the coolant composition significantly increases, in particular, at a low temperature. Accordingly, as a technique of the related art for improving viscosity, a technique of reducing viscosity to improve fluidity at a low temperature has been disclosed (Japanese Patent Application Publication No. 8-183950 (JP 8-183950 A), Japanese Patent Application Publication No. 2010-236064 (JP 2010-236064 A), and Japanese Patent Application Publication No. 9-227859 (JP 9-227859 A)).
However, when the viscosity is reduced, the thickness of a boundary layer between a coolant and a bore wall decreases, and convection is likely to be generated. Therefore, the coolant is likely to absorb heat from the bore wall. As a result, cooling loss increases, and there is a new problem in that fuel efficiency deteriorates. On the other hand, in order to reduce cooling loss by decreasing heat radiation, a configuration of increasing the concentration of a glycol such as ethylene glycol to increase the viscosity of a coolant at a low temperature may be adopted. However, this configuration has a problem in that cooling performance at a high temperature is insufficient, which may cause overheating.
For example, Japanese Patent Application Publication No. 2011-137089 (JP 2011-137089 A), Japanese Patent Application Publication No. 2011-132285 (JP 2011-132285 A), and Japanese Patent Application Publication No. 2011-121991 (JP 2011-121991 A) disclose techniques of improving the viscosity of a lubricant by the addition of a viscosity index improving agent. However, the viscosity index improving agent disclosed herein is added in order to suppress a decrease in viscosity at a high temperature while maintaining fluidity at a low temperature. Accordingly, even when a solution to which the viscosity index improving agent is added is used as a coolant, cooling loss at a low temperature cannot be reduced, and cooling performance at a high temperature cannot be maintained.